1. Field of the Invention
This invention relates to a process for forming, recovering and separating organic acids and particularly relates to a process for separating intermixed polycarboxylic acids which are contaminated with catalytic materials, tarry materials, color bodies or degradation products. This invention specifically relates to a process in which the water extractable, nonvolatile by-products from the air oxidation of cyclohexane are subjected to further oxidation under hydrolytic conditions, esterified in an aqueous solution, extracted and separated into component diesters of polycarboxylic acids, whereby the esters or acids are then separately recovered.
2. Description of the Prior Art
Adipic acid, of course, is used in large quantities for the production of nylon by reaction with alkylene diamines to produce polyamides which are capable of being spun into fibers having a number of well known desirable characteristics. As a result there have been developed a number of processes for the preparation of adipic acid. One of the most preferred processes for making adipic acids in commercial quantities is the two-stage oxidation of cyclohexane, the first stage being a partial oxidation step using air or oxygen to produce partially oxidized products comprising primarily cyclohexanol and cyclohexanone, this mixture often being called "anolone," as well as minor amounts of dicarboxylic acids and dicarboxylic acid precursors, and a substantial amount of unreacted cyclohexane which is recycled in a continuous process. In the second stage of this reaction, the cyclohexanol and cyclohexanone, which are contained in an organic phase after separation from an aqueous phase, are oxidized with nitric acid at elevated temperatures to produce a resulting oxidate mixture comprising a major amount of adipic acid and smaller amounts of other materials including other dicarboxylic acids such as glutaric acid and succinic acid as well as catalytic components and spent nitric acid. The art is also well aware of a number of processes for recovery of the desirable adipic acid product as well as the by-product components. The processes, and various improvements thereon, have been disclosed, for example, in U.S. Pat. Nos. 2,439,513; 2,557,282; 2,791,566; 2,840,607; 2,971,010; and 3,338,959.
In this two-stage oxidation process, it has been found that in the first or air-oxidation stage, there is obtained, in addition to the partial oxidation products comprising cyclohexanol and cyclohexanone, a number of by-products which are not ideal for use in the nitric acid oxidation step. Thus, in this air-oxidation reaction, there is obtained an organic phase containing the desired cyclohexanol and cyclohexanone products as well as cyclohexane starting material and an aqueous phase of undesirable by-product materials comprising a small amount of monobasic acids, a larger amount of dibasic acids, such as oxalic acid, succinic acid, glutaric acid and adipic acid, as well as a considerable amount of oxygenated monobasic acids such as .omega.-hydroxyhexanoic acid and larger amounts of lactones, esters and polymeric esters.
As indicated, a large portion of this aqueous by-product fraction is normally removed from the air oxidation product and the latter, after further purification, is forwarded to the nitric acid oxidation step. A preferred technique for removing a portion of this by-product aqueous fraction from the air oxidation reactor effluent consists essentially of carrying out a water extraction on the reactor effluent or on the reactor effluent from which a portion of the unconverted cyclohexane has been removed. The aqueous phase, after carrying out this extraction, contains a small quantity of cyclohexanol and cyclohexanone and larger amounts of water-soluble and partially water-soluble compounds. Concentration of this aqueous extract by volatilization results in removal of a large portion of the water which can be re-used in the extraction process, and in removal of nearly all of the cyclohexanol and cyclohexanone as well as other volatile compounds. These can then be returned to the purification train of the air oxidation process.
The incorporation of the resulting non-volatile mixture, which, as indicated, contains monobasic and dibasic acids as well as oxygenated monobasic acids, lactones, esters and polymeric esters, directly into the feed of the nitric acid oxidation step of the adipic acid process, will result in an increase in the yield of adipic acid per unit of cyclohexane consumed, but there are practical disadvantages to doing this. Thus, there are inordinate increases in nitric acid consumption, in foaming withing with reaction mixture, in off-gas formation and in production of succinic and glutaric acids. These disadvantages in turn cause a significant reduction in the production capacity of an adipic acid plant of a given size and thus outweigh the increase in adipic acid yield. On the other hand, purification of the adipic acid or other valuable compounds present in the concentrated aqueous solution of extractable nonvolatile by-products is also impractical due to the complex nature of the mixture. Therefore, as a result of the difficulties mentioned, this by-product mixture is ordinarily treated as waste, the disposition of which is in itself an expensive process.
A recent patent, U.S. Pat. No. 3,365,490, is concerned with this problem and discloses a process for the nitric acid oxidation of this by-product material apart from the nitric acid oxidation of the main product resulting from the air-oxidation process and using reaction conditions different from those used in the nitric acid oxidation step. While this prior patent procedure provides increased yields of adipic acid relative to yields of the other dibasic acids, nevertheless, the dibasic acids produced by this process are difficult to separate, both from the reaction mixture and also from each other, due to the presence of relatively large amounts of succinic acid and glutaric acid which are produced in the process. Basically, this patent provides an improvement which comprises concentrating the effluent from the air-oxidizer to about 60-80% cyclohexane, extracting a useful portion of the non-volatile by-products from the organic phase with water, concentrating the aqueous extract and employing the resulting steam to steam-distill the cyclohexanol and cyclohexanone away from the non-useful portion of the non-volatile residue remaining in the organic phase and oxidizing the residue from the aqueous extract to adipic acid with aqueous 30-70% nitric acid at a low temperature (35.degree.-60.degree. C.) in the presence of added NO.sub.2. The resultant product is a mixture of the dicarboxylic acids, which, as pointed out above, is still difficult to separate. Quite obviously, therefore, this patent does not provide means by which the other acids present can be recovered and the yield of desirable products maximized.
There are also processes known in the art by which a mixture of dicarboxylic acids can be treated for separation of the acids from each other. Thus, U.S. Pat. No. 2,824,123 describes a process for the separation of dicarboxylic acids by forming the diesters thereof in a conventional manner and distilling the esters by conventional techniques to effect partial separation and cooling the distilled fractions to crystallize esters of the acids. Thus, this patent describes a process for the separation of dicarboxylic acids one from another but not where such acids are contained in an oxidate stream. Similarly, companion U.S. Pat. No. 2,824,122 describes a process for the selective removal of dibasic acids from aqueous streams by converting the acids to ester derivatives with a water-immiscible alcohol and removing the water therefrom during the esterification step. In this latter process an azeotroping agent is often added to the mixture to increase the efficiency of water removal. Neither of these patents however discloses processes by which the polycarboxylic acids resulting from the nitric acid oxidation can be separated from the aqueous stream and from each other so as to recover all desirable and useful products.
It is accordingly clear that a need remains in the art for processes by which valuable products can be effectively recovered from the aqueous by-product extract obtained from the air-oxidation of cyclohexane which can be employed to maximize the production of adipic acid and result in increased efficiency of the overall process.